The present invention relates to an electron beam device making up an interferometer using an electron biprism.
An electron biprism is used in explanations of the present specification. The electron biprism is a device in electron optics having the same action as that of Fresnel's biprism in optics and is classified into two types; electric field type and magnetic field type. Of the two, the one more widely used is the electric field type electron biprism and has such a shape as shown in FIG. 1. That is, the electric field type electron biprism is made up of a filament electrode 9 in the center and parallel flat plate type grounded electrodes 99 held so as to sandwich the electrode. For example, when a positive voltage is applied to the central filament electrode 9, as shown in FIG. 1, electron beams passing in the vicinity of the filament electrode sense the potential of the central filament electrode and are deflected in mutually opposed directions (see trajectories 24 of the electron beams). The greater the distance from the central filament electrode, the smaller the potential acting on the electron beams becomes, but since the acting spatial range is long, the deflection angle of the electron beam is proportional to an applied voltage to the filament electrode irrespective of the position of incidence as a consequence. That is, assuming α is the deflection angle of the electron beam by the electron biprism, there is a simple relationship expressed as α=kVf using an applied voltage Vf to the central filament electrode and a deflection factor k. The fact that the deflection angle α of the electron beam is not dependent on the position of incidence is an important feature as the optical device and a plane wave (wavefront 22) remains as is with only the propagation direction deflected, and passed away from the electron biprism. This corresponds to the effect of a biprism combining two prisms in optics, and is called an “electron biprism.” The electron biprism using a potential for deflecting an electron beam is called an “electric field type electron biprism” and the electron biprism using Lorentz force between the magnetic field and electron beam is called a “magnetic field type electron biprism.” The present specification will give explanations using the electric field type electron biprism. However, the present invention can be configured by any device, whether electric field type or magnetic field type, if the device at least allows interference with an electron beam as the electron biprism, and is not limited to the electric field type electron biprism used for explanations. Furthermore, when the present specification describes an “electron biprism,” this means an entire electron biprism as an electron beam deflector including a central filament electrode in a broad sense and describes a “central filament electrode of an electron biprism” as a general rule when the specification refers to an exact position in an optical system.
The electron biprism is a device indispensable to creating electron beam interference in an electron beam with no beam splitter such as a half mirror in optics. As is also clear from FIG. 1, this is attributable to the function of separating a wavefront 22 of one electron beam into two wavefronts and deflecting the wavefronts in the mutually opposed directions. As a result, the two separated electron beams after passing through the electron biprism are superimposed behind the electron biprism and produce an interference fringe 8. Such an electron optical system is generically called an “electron beam interference optical system.”
<Single-Biprism Electron Interferometer>
As shown in FIG. 2, a most common electron beam interferometer represented by electron holography disposes one electron biprism between an objective lens 55 and an image plane 71 of a specimen, applies a positive voltage to a central filament electrode 9, thereby causes an electron beam that has passed through the specimen (object wave 21: electron beam that passes on the left of the central filament electrode, shown with hatching in FIG. 2) and an electron beam that passes on the side where the specimen does not exist (reference wave 23: electron beam that passes on the right of the central filament electrode in FIG. 2) to be superimposed together and obtains an interferogram (31 and 8: image resulting from superimposition of the interference fringe 8 on the specimen image 31). In this case, there is a predetermined relationship between an interference fringe spacing s and an interference width W and these are expressed by the following expressions as an interference fringe spacing sobj and interference width Wobj backprojected onto the specimen plane.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      s          obj                =                              1            M                    ⁢                                    D              ⁢                                                          ⁢              λ                                      2              ⁢                                                          ⁢                              α                ⁡                                  (                                      D                    -                    L                                    )                                                                                        (        1        )                                [                  Expression          ⁢                                          ⁢          2                ]                                                                      W          obj                =                              1            M                    ⁢                      (                                          2                ⁢                                                                  ⁢                α                ⁢                                                                  ⁢                L                            -                                                D                                      D                    -                    L                                                  ⁢                                  d                  fi                                                      )                                              (        2        )            
Here, α (rad) is a deflection angle of the electron beam by the electron biprism. Other reference characters mainly relate to distances between elements such as the object, lens and image in the optical system and are shown in FIG. 2. That is, “a” denotes a distance between a specimen plane 3 (object plane) and the objective lens 55, “b” denotes a distance between the objective lens 55 and the image plane 71 of the specimen, “D” denotes a distance from an image plane 11 of an electron source below the objective lens to the image plane 71 of the specimen by the objective lens 55 and “L” denotes a distance from the central filament electrode 9 of the electron biprism to the image plane 71 of the specimen. Furthermore, “M” denotes a magnification M=b/a of this optical system and “dfi” denotes a diameter of the central filament electrode 9.
As is clear from Expressions (1) and (2), both the interference fringe spacing sobj and interference width Wobj are functions of the deflection angle α of the electron beam and cannot be controlled independently of each other only by an applied voltage Vf to the central filament electrode.
When a charged particle beam including an electron beam passes through the electromagnetic lens, a rotation of an azimuth centered on the optical axis occurs in the charged particle beam, but FIG. 2 omits this rotation and describes an equivalent plane including the optical axis as an electron optical system. Furthermore, suppose the central filament electrode 9 is disposed perpendicular to the plane of the sheet, the cross section of the electrode is shown with a small circle and parallel flat plate type grounded electrodes on both sides of the central filament electrode 9 are omitted. The omission of the rotation of this azimuth in the figure of the optical system and the omission of the display of the central filament electrode or the like will be the same in subsequent figures unless specified otherwise. Moreover, in the figure showing the optical system in the present specification, since the pre-field of an objective lens system 5 which will be described later is an important element unless specified otherwise, the optical system is separated into two portions of a pre-field lens 51 and the objective lens 55 or separated into three portions of the pre-field lens 51, a middle pre-field lens 53 (see FIGS. 12 to 14) and the objective lens 55.
<Double-Biprism Electron Interferometer>
It is a multi-biprism electron interferometer that has been developed so as to overcome the disadvantage of the single-biprism electron interferometer that the interference fringe spacing s and interference width W cannot be controlled independently of each other. FIG. 3 shows a double-biprism interference optical system which is the simplest configuration of the multi-biprism electron interferometer.
In this optical system, an upper electron biprism 91 is disposed on a first image plane 71 of a specimen downstream of an objective lens and a lower electron biprism 95 is disposed between an image plane 12 of a source imaged by a first intermediate lens 61 disposed downstream of the first image plane 71 in the traveling direction of the electron beam and a second image plane 72 of the specimen downstream of the first intermediate lens and also in the shadow of a central filament electrode 9 of the upper biprism (shown by dark hatching in FIG. 3). In FIG. 3, both central filament electrodes are disposed perpendicular to the plane of the sheet. The two parameters of the interferogram (31 and 8) in this configuration; interference fringe spacing s and interference width W are back-projected onto the specimen plane and are expressed as an interference fringe spacing sobj and interference width Wobj by the following expressions.
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      s          obj                =                              1                          M              U                                ⁢                      1                          M              L                                ⁢                                    λ              ⁢                                                          ⁢                              D                L                                                    2              ⁢                              {                                                                                                    b                        2                                                                    a                        2                                                              ⁢                                          D                      U                                        ⁢                                          α                      U                                                        +                                                            (                                                                        D                          L                                                -                                                  L                          L                                                                    )                                        ⁢                                          α                      L                                                                      }                                                                        (        3        )                                [                  Expression          ⁢                                          ⁢          4                ]                                                                      W          obj                =                              1                          M              U                                ⁢                      (                                                            1                                      M                    L                                                  ⁢                2                ⁢                                                                  ⁢                                  L                  L                                ⁢                                  α                  L                                            -                              d                U                                      )                                              (        4        )            where, “αU” is a deflection angle of the electron beam by the upper electron biprism 91 and “αL” is a deflection angle of the lower electron biprism 95. Furthermore, other characters in the expression mainly relate to distances between the respective components in the optical system such as the object, lens and image and are shown in FIG. 3. That is, “aU” is a distance between a specimen plane 3 (object plane) and the objective lens 55, “bU” is a distance between an objective lens 55 and the first image plane 71 of the specimen, “aL” is a distance between the first image plane 71 of the specimen (object plane of the first intermediate lens) and the first intermediate lens 61, “bL” is a distance between the first intermediate lens 61 and the second image plane 72 of the specimen, “a2” is a distance between an image plane 11 of the electron source below the objective lens and the first intermediate lens 61, “b2” is a distance between the first intermediate lens 61 and an image plane 12 of the electron source below the first intermediate lens, “DU” is a distance from the image plane 11 of the electron source below the objective lens to the image plane 71 of the specimen by the objective lens, “DL” is a distance from the image plane 12 of the electron source below the first intermediate lens to the image plane 72 of the specimen (second image plane of the specimen) by the first intermediate lens and “LL” is a distance from the central filament electrode 95 of the lower electron biprism to the second image plane 72 of the specimen. Furthermore, “MU” and “ML” are magnifications MU=bU/aU and ML=bL/aL of this imaging optical system respectively and “dU” is a diameter of the central filament electrode of the upper electron biprism.
As is clear from Expressions (3) and (4), the interference fringe spacing sobj is expressed as a function of αU and the interference width Wobj is expressed as a function of αL and αL, and though not completely independent of each other, these parameters can be controlled effectively independently of each other by determining a sequence of operations for acquiring an interferogram as:    (1) Defining a required interference width by adjusting the applied voltage to the lower electron biprism 95, and    (2) Obtaining a required interference fringe spacing by adjusting the applied voltage to the upper electron biprism 91.Now if the central filament electrode 95 of the lower electron biprism is disposed on the image plane 12 of the source by the first intermediate lens in FIG. 3, that is, when a parameter DL−LL=0, s and W can be controlled completely independently of each other (see JP-A-2005-197165, JP-A-2007-115409 and JP-A-2006-216345).<Triple-Biprism Electron Interferometer>
It is a triple-biprism electron optical system that the optical system of the double electron biprism has been further developed and one example thereof has such a configuration as shown in FIG. 4. An upper electron biprism 91 is disposed on a first image plane 71, a middle electron biprism 93 is disposed on a second image plane 72 and a lower electron biprism 95 is disposed between a second intermediate lens 62 and a third image plane 73. The azimuth of the central filament electrode 93 of the middle electron biprism is orthogonal to the upper electron biprism 91. In FIG. 3, the central filament electrode 91 of the upper electron biprism is expressed with a horizontally oriented line assumed to be disposed within the plane represented by the plane of the sheet and the central filament electrode 93 of the middle electron biprism is expressed with a cross section of the electrode (same as FIG. 2 and FIG. 3) assumed to be disposed perpendicular to the plane represented by the plane of the sheet. Furthermore, since the azimuth of the central filament electrode 95 of the lower electron biprism forms an angle of 45° with the upper and middle filament electrodes, the azimuth is expressed with a short horizontal line in FIG. 4. The position of the triple-electron biprism in the optical system and relative azimuths of the respective central filament electrodes are not limited to those in FIG. 4 and can take various positional relationships and azimuthal relationships. Though expressions are not described, this optical system can control not only the interference fringe spacing s and interference width W but also an azimuth θ of interference fringe independently of each other (see JP-A-2006-313069).
As shown above, the problems with control on parameters (interference fringe spacing s, interference width W and azimuth θ) of the interferogram are solved by the multi-biprism electron interferometer.
The multi-biprism electron interferometer adopts a configuration with a plurality of electron biprisms arranged in an imaging optical system and the effect of electron beam deflection by the biprisms in the optical system is achieved by combining lenses corresponding to the respective biprisms and operating the biprisms in association therewith. Therefore, as is clear from comparisons between FIG. 2 and FIG. 3 or FIG. 4, the specimen is finally imaged as an appropriate interferogram through the objective lens 55 right below the specimen and first intermediate lens 61 further therebelow in the double-biprism electron interferometer and through the objective lens 55, first intermediate lens 61 and lowest second intermediate lens 62 in FIG. 4 in the triple-biprism electron interferometer. The interferogram is further magnified/demagnified by electromagnetic lenses downstream in the traveling direction of the electron beam (not shown in FIGS. 2, 3 and 4) and finally observed and recorded as an interferogram. FIG. 5 shows an example of the optical system of a conventional transmission electron microscope when constructing a double-biprism electron interferometer. According to convention, in FIG. 5, the effects of the pre-field of the objective lens are described collectively in a condenser optical system 4 and shown in a single objective lens system 5.
As shown in FIG. 5, the conventional transmission electron microscope is constructed of a total of five imaging lens systems; the objective lens system 5 and four magnifying lens systems (made up of lenses 61 to 64). When a double-biprism electron interferometer is constructed, operating conditions of the objective lens system 5 and the first intermediate lens 61 are uniquely determined by adjusting the positions or the like of an imaging position 31 of a specimen 3 and an electron biprism 91. Furthermore, the second projection lens 64 at the final stage of the imaging lens is often used specialized for projecting of the final image to a recording system 79 such as a film. As a result, the second intermediate lens 62 and first projection lens 63 are the only electromagnetic lenses that can secure the degree of operational flexibility for an interferogram (35 and 8) such as a change of image magnification. That is, in order to construct a multi-biprism electron interferometer after securing the degree of operational flexibility for the interferogram (35 and 8), it is necessary to additionally configure not only the electron biprism but also the electromagnetic lens to the imaging optical system of the conventional electron microscope. For example, when an electromagnetic lens is added, the size of the entire device increases due to the addition of the electromagnetic lens. Furthermore, a control mechanism for operating the added electromagnetic lens also needs to be newly added, which complicates control.